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| United States Patent |
6,184,300
|
|
Shimokawa
, et al.
|
February 6, 2001
|
Bowl-shaped polymer particles, aqueous dispersion of polymer particles,
processes for producing these, and thermal recording material
Abstract
Bowl-shaped polymer particles characterised by having a bowl-like shape as
is formed by partly cutting along a plane each spherical hollow polymer
particle having a void in the inside, and in which the cross-section
perpendicular to the cut plane and passing through the center of the
particle has a shape as is formed by partly cutting a double circle along
a straight line, and the lengths H of the perpendiculars from the outer
arc of the double circle in the cross-section to the straight line have a
maximum value H.sub.max equal to or larger than the radius of the outer
circle of the double circle. The bowl-shaped polymer particles have a
multi-layer structure, and are produced by multi-stage polymerization in
which monomer mixtures are successively polymerized in a manner such that
the proportion of a monomer having an acidic group gradationally decreases
and by treating the resultant polymer with a base. The bowl-shaped polymer
particles are useful as a heat-insulating material, organic pigment, or
opacifying agent, and especially as a heat-insulating interlayer to be
formed in a thermal recording material.
| Inventors:
|
Shimokawa; Yoshiharu (Yokohama, JP);
Ishizu; Osamu (Kawasaki, JP)
|
| Assignee:
|
Nippon Zeon Co., Ltd. (Tokyo, JP)
|
| Appl. No.:
|
367048 |
| Filed:
|
August 6, 1999 |
| PCT Filed:
|
February 9, 1998
|
| PCT NO:
|
PCT/JP98/00526
|
| 371 Date:
|
August 6, 1999
|
| 102(e) Date:
|
August 6, 1999
|
| PCT PUB.NO.:
|
WO98/34969 |
| PCT PUB. Date:
|
August 13, 1998 |
Foreign Application Priority Data
| Feb 07, 1997[JP] | 9-039833 |
| Feb 07, 1997[JP] | 9-039834 |
| Current U.S. Class: |
525/242 |
| Intern'l Class: |
C08F 251/00 |
| Field of Search: |
525/242
|
References Cited
| Foreign Patent Documents |
| 2-14222 | Jan., 1990 | JP.
| |
| 5-222108 | Aug., 1993 | JP.
| |
| WO98/34969 | Feb., 1997 | WO.
| |
Primary Examiner: Boykin; Terressa M.
Attorney, Agent or Firm: Armstrong, Westerman, Hattori, McLeland & Naughton
Claims
What is claimed is:
1. Bowl-shaped polymer particles characterized by having a bowl-like shape
as is formed by partly outting along a plane each spherical hollow polymer
particle having a void in the inside, and in which the cross-section
perpendicular to the cut plane and passing through the center of the
particle has a shape as is formed by partly cutting a double circle along
a straight line, and the lengths H of the perpendiculars from the outer
arc of the double circle in the cross-section to the straight line have a
maximum value H.sub.max equal to or larger than the radius of the outer
circle of the double circle.
2. The bowl-shaped polymer particles according to claim 1, which have an
average maximum diameter in the range of 0.3 to 5 .mu.m.
3. The bowl-shaped polymer particles according to claim 1, wherein the
ratio of the average outer diameter of the circular brim of the bowl to
the average maximum diameter of the polymer particles is in the range of
25 to 100%.
4. The bowl-shaped polymer particles according to claim 1, wherein the
ration of the radial thickness of the circular brim of the bowl is in the
range of 10 to 30% of the average maximum diameter of the particles.
5. A process for producing bowl-shaped polymer particles, characterized by
comprising the steps of:
(1) copolymerizing a monomer mixture comprising 30 to 65% by weight of an
acidic group-containing monomer and 70 to 35% by weight of a monomer
copolymerizable therewith to prepare a nucleus polymer particle;
(2) copolymerizing a monomer mixture comprising 10 to 35% by weight of an
acidic group-containing monomer and 90 to 65% by weight of a monomer
copolymerizable therewith in the presence of the nuclear polymer particle
to prepare a core polymer particle substantially surrounding the nucleus
polymer particle, wherein the content of the acidic group-containing
monomer units in the core polymer particle is equal to or smaller than the
content of the acid group-containing monomer units in the nucleus polymer
particle;
(3) copolymerizing a monomer mixture comprising 1 to 12% by weight of an
acidic group-containing monomer and 99 to 88% by weight of a monomer
copolymerizable therewith in the presence of the nucleus/core polymer
particle to form at least one intermediate polymer layer which
substantially surrounds the nucleus/core polymer particle;
(4) polymerizing an aromatic vinyl monomer alone or copolymerizing a
monomer mixture comprising up to 0.5% by weight of an acidic
group-containing monomer and at least 99.5% by weight of an aromatic vinyl
monomer copolymerizable therewith in the presence of the polymer particle
which is prepared in step (3) and comprising the intermediate polymer
layer substantially surrounding the nucleus/core polymer particle, to
prepare an outer polymer layer which substantially surrounds the polymer
particle prepared in step (3);
(5) adding a base to an aqueous dispersion of the polymer particle which is
prepared in step (4) and has a multi-layer structure comprising at least
four layers, to adjust the pH value of the aqueous polymer dispersion to
at least 7;
(6) adding an aromatic vinyl monomer or a monomer mixture comprising at
least 90% by weight of an aromatic vinyl monomer and not larger than 10%
by weight of a monomer copolymerizable copolymerize the monomer mixture in
the presence of the polymer particle with the multi-layer structure,
prepared in step (5), to form an outermost polymer layer surrounding the
outer polymer layer prepared in step (4); and then,
(7) drying the thus-prepared aqueous polymer dispersion.
6. The process for producing bowl-shaped polymer particles according to
claim 5 wherein, in step (1), a monomer mixture comprising 30 to 55% by
weight of an acidic group-containing monomer and 70 to 45% by weight of a
monomer copolymerizable therewith is copolymerized to prepare a nucleus
polymer particle.
7. The process for producing bowl-shaped polymer particles according to
claim 5 wherein, in step (2), a monomer mixture comprising 15 to 35% by
weight of an acidic group-containing monomer and 85 to 65% by weight of a
monomer copolymerizable therewith is copolymerized in the presence of the
nucleus polymer particle to prepare a core polymer particle substantially
surrounding the nucleus polymer particle.
8. The process for producing bowl-shaped polymer particles according to
claim 5 wherein, in step (2), the monomer mixture for forming the core
polymer particle is used in an amount such that the amount of the nucleus
polymer particle is in the range of 1 to 25 parts by weight based on 100
parts by weight of the core polymer particle.
9. The process for producing bowl-shaped polymer particles according to
claim 5 wherein the nucleus/core polymer particle prepared in step (2) has
a particle diameter in the range of 150 to 500 nm.
10. The process for producing bowl-shaped polymer particles according to
claim 5 wherein, in step (3), a monomer mixture comprising 2 to 10% by
weight of an acidic group-containing monomer and 98 to 90% by weight of a
monomer copolymerizable therewith is copolymerized in the presence of the
nucleus/core polymer particle to form at least on intermediate polymer
layer which substantially surrounds the nucleus/core polymer particle.
11. The process for producing bowl-shaped polymer particles according to
claim 5 wherein, in step (3), the monomer mixture for forming the
intermediate polymer layer is used in an amount such that the ratio of the
core polymer particle to the intermediate polymer layer is in the range of
5/95 to 60/40 by weight.
12. The process for producing bowl-shaped polymer particles according to
claim 5, wherein the polymer particle, prepared in step (3) and composed
of the intermediate polymer layer substantially surrounding the
nucleus/core polymer particle, has a particle diameter in the range of 300
to 900 nm; and the ratio by weight of the nucleus/core polymer particle to
the polymer particle, prepared in step (3) and composed of the
intermediate polymer layer substantially surrounding the nucleus/core
polymer particle, is at least 0.05.
13. The process for producing bowl-shaped polymer particles according to
claim 5 wherein, in step (4), the monomer or monomer mixture for preparing
the outer polymer layer is used in an amount such that the ratio by weight
of the polymer particle, prepared in step (3) and composed of the
intermediate polymer layer substantially surrounding the nucleus/core
polymer particle, to the polymer particle, prepared in step (4) and having
the outer polymer layer substantially surrounding the polymer particle
prepared in step (3) is in the range of 0.4 to 0.6; and that the ratio by
weight of the nucleus/core polymer particle to the polymer particle,
prepared in step (4) and having the outer polymer layer is at least 0.02.
14. The process for producing bowl-shaped polymer particles according to
claim 5 wherein, in step (5), an alkali metal hydroxide is added as a base
to the aqueous dispersion of the polymer particle, prepared in step (4)
and having a multi-layer structure comprising at least four layers, to
adjust the pH value of the aqueous polymer dispersion to at least 7.
15. The process for producing bowl-shaped polymer particles according to
claim 5 wherein, after step (5), 0.01 to 40 parts by weight, based on 100
parts by weight of the sum of the monomers used in steps (1) to (4), of an
acidic group-containing monomer is added to the base-added aqueous
dispersion of the polymer particle to lower the pH value thereof below 7.
16. The process for producing bowl-shaped polymer particles according to
claim 5 wherein, in step (6), the monomer or monomer mixture for preparing
the outermost polymer layer is used in an amount such that the ratio by
weight of the polymer particle, prepared in step (4) and having the outer
polymer layer substantially surrounding the polymer particle prepared in
step (3), to the outermost polymer layer is in the range of 100/10 to
50/100.
17. The process for producing bowl-shaped polymer particles according to
claim 5, wherein the acidic group-containing monomer used in steps (1),
(2), (3) and (4) is selected from monocarboxylic acids and monoesters of
dicarboxylic acids.
18. The process for producing bowl-shaped polymer particles according to
claim 5, wherein the aromatic vinyl monomer used in steps (4) and (6) is
styrene.
19. The process for producing bowl-shaped polymer particles according to
claim 5 wherein, in the drying step (7), the aqueous polymer dispersion is
subjected to spray drying.
20. An aqueous dispersion of polymer particles, which is capable of, when
the aqueous dispersion is dried, producing the bowl-shaped polymer
particles as claim in claim 1.
21. A process for preparing the aqueous polymer particle dispersion capable
of producing the bowl-shaped polymer particles, as claimed in claim 20,
which is characterized by comprising the steps of:
(1) copolymerizing a monomer mixture comprising 30 to 65% by weight of an
acidic group-containing monomer and 70 to 35% by weight of a monomer
copolymerizable therewith to prepare a nucleus polymer particle;
(2) copolymerizing a monomer mixture comprising 10 to 35% by weight of an
acidic group-containing monomer and 90 to 65% by weight of a monomer
copolymerizable therewith in the presence of the nuclear polymer particle
to prepare a core polymer particle substantially surrounding the nucleus
polymer particle, wherein the content of the acidic group-containing
monomer units in the core polymer particle is equal to or smaller than the
content of the acid group-containing monomer units in the nucleus polymer
particle;
(3) copolymerizing a monomer mixture comprising 1 to 12% by weight of an
acidic group-containing monomer and 99 to 88% by weight of a monomer
copolymerizable therewith in the presence of the nucleus/core polymer
particle to form at least one intermediate polymer layer which
substantially surrounds the nucleus/core polymer particle;
(4) polymerizing an aromatic vinyl monomer alone or copolymerizing a
monomer mixture comprising up to 0.5% by weight of an acidic
group-containing monomer and at least 99.5% by weight of an aromatic vinyl
monomer copolymerizable therewith in the presence of the polymer particle
which is prepared in step (3) and comprising the intermediate polymer
layer substantially surrounding the nucleus/core polymer particle, to
prepare an outer polymer layer which substantially surrounds the polymer
particle prepared in step (3);
(5) adding a base to an aqueous dispersion of the polymer particle which is
prepared in step (4) and has a multi-layer structure comprising at least
four layers, to adjust the pH value of the aqueous polymer dispersion to
at least 7; and then
(6) adding an aromatic vinyl monomer or a monomer mixture comprising at
least 90% by weight of an aromatic vinyl monomer and not larger than 10%
by weight of a monomer copolymerizable therewith, to polymerize the
aromatic vinyl monomer or copolymerize the monomer mixture in the presence
of the polymer particle with the multi-layer structure, prepared in step
(5), to form an outermost polymer layer surrounding the outer polymer
layer prepared in step (4).
22. A thermal recording material having a thermal recording layer formed on
a support, characterized in that an intermediate layer comprising
heat-insulating polymer particles is formed between the thermal recording
layer and the support, said polymer particles being the bowl-shaped
polymer particles as claimed in claim 1.
Description
TECHNICAL FIELD
This invention relates to bowl-shaped polymer particles; an aqueous
dispersion of polymer particles capable of, when it is dried, forming the
bowl-shaped polymer particles; and processes for producing the bowl-shaped
polymer particles and the aqueous polymer particle dispersion.
The bowl-shaped polymer particles of the present invention are used as a
heat-insulating material, an organic pigment and an opacifying agent, and
as ingredients to be incorporated in a material for forming a
heat-insulating intermediate layer of a thermal recording material, an
aqueous coating composition, a coating color for paper, and others.
BACKGROUND ART
It has been hitherto known that hollow polymer particles cause irregular
reflection of irradiated light due to the presence of a void in the
interior thereof, and thus, when the hollow polymer particles are coated
on a material, whiteness of the material is enhanced and the material is
made opaque.
It is considered that, as the diameter of the hollow polymer particles
becomes large, the opacifying effect becomes prominent.
Polymer particles having a special shape, other than the hollow polymer
particles, also are known as exhibiting an opacifying effect. For example,
in Japanese Unexamined Patent Publication (hereinafter abbreviated to
"JP-A") No. H5-222108, polymer particles having at least one through-hole
extending from the particle surface to the interior thereof, which is
formed when the particles are dried, and a process for producing the
polymers are described. It is described therein that the polymer particles
are usable as a coating composition and an organic pigment for paper
coating color. However, the particle size of the polymer particles,
produced by this process, are small, i.e., at most about 0.2 .mu.m, and
thus, the opacifying effect of the pigment is poor.
A thermal recording material has been proposed in JP-A H2-164580, which has
a heat-insulating intermediate layer between a support and a thermal
recording layer, wherein the intermediate layer contains porous and
non-spherical hollow polymer particles having undulations on the surface
which are formed by allowing hollow polymer particles to shrink. These
non-spherical hollow polymer particles exhibit good thermal response, but
have poor adhesion between the thermal recording layer and the support.
Another thermal recording material has been proposed in JP-A H5-573, which
has a heat-insulating intermediate layer between a support and a thermal
recording layer, wherein the intermediate layer contains spherical or
ellipsoidal plastic hollow particles. The plastic hollow particles are
fine foamed plastic hollow particles having an average particle diameter
of 2.0 to 20 .mu.m, preferably 3 to 10 .mu.m. It is however difficult to
prepare fine foamed plastic hollow particles which are uniform and have a
narrow particle size distribution.
A further thermal recording material has been proposed in JP-A H2-57382,
which has a heat-insulating intermediate layer between a support and a
thermal recording layer, wherein the intermediate layer contains fine
hollow polymer particles having a particle diameter of not larger than 5
.mu.m, preferably 0.1 to 3 .mu.m. These hollow polymer particles exhibit a
poor effect for enhancement of thermal response, and further have problems
of causing sticking and deposition of tailings on a thermal head.
Emulsified vinyl polymer particles have been proposed in JP-A-H2-14222,
which have a flat shape, and at least one major surface of which is
concave. It is described therein that these polymer particles are used as
an additive for a paint, a paper coating color, or a coating color for
information recording paper and other materials. It is however
indispensable for the preparation of the polymer particles to conduct
emulsion polymerization using a non-aqueous organic solvent such as an
aliphatic hydrocarbon, and, after the completion of polymerization, to
remove the organic solvent from the reaction system. Due to the flat
shape, the polymer particles exhibit poor opacifying effect, and, where
they are used for a thermal recording paper, they exhibit a poor
heat-insulating effect.
DISCLOSURE OF INVENTION
An object of the present invention is to provide polymer particles having a
special shape, which exhibit excellent heat-insulating effect,
whiteness-enhancing effect, opacifying effect, and other effects.
Another object of the present invention is to provide a process for stably
producing the polymer particles of a special shape.
Still another object of the present invention is to provide a thermal
recording material, which exhibits excellent thermal response, high
dynamic sensitivity, and good reproducibility of printed letter images,
i.e., dots, and in which problems of causing sticking and deposition of
tailings on a thermal head are minimized.
In one aspect of the present invention, there is provided bowl-shaped
polymer particles characterized by having a bowl-like shape as is formed
by partly cutting along a plane each spherical hollow polymer particle
having a void in the inside, and in which the cross-section perpendicular
to the cut plane and passing through the center of the particle has a
shape as is formed by partly cutting a double circle along a straight
line, and the lengths H of the perpendiculars from the outer arc of the
double circle in the cross-section to the straight line have a maximum
value H.sub.max equal to or larger than the radius of the outer circle of
the double circle.
In another aspect of the present invention, there is provided an aqueous
dispersion of polymer particles, which is capable of, when the aqueous
dispersion is dried, producing the above-mentioned bowl-shaped polymer
particles.
In still another aspect of the present invention, there is provided a
process for producing bowl-shaped polymer particles, characterized by
comprising the steps of:
(1) copolymerizing a monomer mixture comprising 30 to 65% by weight of an
acidic group-containing monomer and 70 to 35% by weight of a monomer
copolymerizable therewith to prepare a nucleus polymer particle;
(2) copolymerizing a monomer mixture comprising 10 to 35% by weight of an
acidic group-containing monomer and 90 to 65% by weight of a monomer
copolymerizable therewith in the presence of the nuclear polymer particle
to prepare a core polymer particle substantially surrounding the nucleus
polymer particle, wherein the content of the acidic group-containing
monomer units in the core polymer particle is equal to or smaller than the
content of the acid group-containing monomer units in the nucleus polymer
particle;
(3) copolymerizing a monomer mixture comprising 1 to 12% by weight of an
acidic group-containing monomer and 99 to 88% by weight of a monomer
copolymerizable therewith in the presence of the nucleus/core polymer
particle to form at least one intermediate polymer layer which
substantially surrounds the nucleus/core polymer particle;
(4) polymerizing an aromatic vinyl monomer alone or copolymerizing a
monomer mixture comprising up to 0.5% by weight of an acidic
group-containing monomer and at least 99.5% by weight of an aromatic vinyl
monomer copolymerizable therewith in the presence of the polymer particle
which is prepared in step (3) and comprising the intermediate polymer
layer substantially surrounding the nucleus/core polymer particle to
prepare an outer polymer layer which substantially surrounds the polymer
particle prepared in step (3);
(5) adding a base to an aqueous dispersion of the polymer particle which is
prepared in step (4) and has a multi-layer structure comprising at least
four layers, to adjust the pH value of the aqueous polymer dispersion to
at least 7;
(6) adding an aromatic vinyl monomer or a monomer mixture comprising at
least 90% by weight of an aromatic vinyl monomer and not larger than 10%
by weight of a monomer copolymerizable therewith, to polymerize the
aromatic vinyl monomer or copolymerize the monomer mixture in the presence
of the polymer particle with the multi-layer structure, prepared in step
(5), to form an outermost polymer layer surrounding the outer polymer
layer prepared in step (4); and then,
(7) drying the thus-prepared aqueous polymer dispersion.
In a further aspect of the present invention, there is provided a process
for producing the above-mentioned aqueous dispersion of polymer particles,
which is capable of, when the aqueous dispersion is dried, producing the
bowl-shaped polymer particles; which process is characterized by
comprising the above-mentioned steps (1) through (6).
In a still further aspect of the present invention, there is provided a
thermal recording material having a thermal recording layer formed on a
support, characterized in that an intermediate layer comprising
heat-insulating polymer particles is formed between the thermal recording
layer and the support; said polymer particles being the bowl-shaped
polymer particles having a bowl-like shape as is formed by partly cutting
along a plane each spherical hollow polymer particle having a void in the
inside, and in which the cross-section perpendicular to the cut plane and
passing through the center of the particle has a shape as is formed by
partly cutting a double circle along a straight line, and the lengths H of
the perpendiculars from the outer arc of the double circle in the
cross-section to the straight line have a maximum value H.sub.max equal to
or larger than the radius of the outer circle of the double circle.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a cross-section which is perpendicular to the opening plane
and passes through the center of the bowl-shaped polymer particle of the
present invention.
FIG. 2 is a scanning electron microscope (SEM) photograph of a bowl-shaped
polymer particle obtained in Example 1.
BEST MODE FOR CARRYING OUT THE INVENTION
I. Bowl-Shaped Polymer Particle
The bowl-shaped polymer particle of the present invention has a shape as is
obtained by partly cutting along a plane approximately spherical hollow
polymer particle having a void (i.e., vacant space) in the inside. A
cross-section perpendicular to the cut plane and passing through the
center of the particle has a shape, as illustrated in FIG. 1, as is formed
by partly cutting a double circle along a straight line "m". In the
cross-section shown in FIG. 1, among lengths H of the perpendiculars drawn
from arbitrary points on the outer arc of the cross-section to the
straight line m, the maximum length H.sub.max is equal to or larger than
the radius (i.e., half of the diameter D) of the outer circle "p" of the
double circle. In other words, the outer diameter of the bowl-shaped
polymer particle is approximately the same as the outer diameter of a
spherical hollow polymer particle as immediately before the formation of
bowl-shape. The shape of the bowl-shaped polymer particle is a half of the
spherical hollow polymer particle as immediately before the formation of
bowl-shape, or larger than the half, namely, the shape is intermediate
between a hemisphere and a sphere. Thus, the ratio of the outer diameter
"D" of the bowl-shaped polymer particle to the maximum height H.sub.max is
larger than 1, and equal to 2 or smaller (i.e., 1<D/H.sub.max.ltoreq.2).
The wall of the bowl-shaped polymer particle becomes thick and inwardly
overhang to a small degree. The wall thickness of the bowl-shaped polymer
particle is about two times of that of the hollow polymer particles before
the formation of the bowl-shape. The wall thickness in the vicinity of
brim of the bowl-shaped polymer particle is about 10% to about 30% of the
outer diameter D of the particle. The bowl-shaped polymer particle may
either have a thin fattened void (i.e., a very thin vacant space)
originating from the void in the inside of the hollow polymer particle
before the formation of bowl-shape, or be solid without any vacant space.
The average maximum diameter of the bowl-shaped polymer particle is
generally in the range of 0.3 to 5 .mu.m, preferably 0.5 to 3 .mu.m. The
ratio of the average outer diameter of the opening brim (i.e., diameter of
the brim including the wall thickness) of the bowl-shaped polymer particle
to the average maximum diameter of the particle is generally in the range
of 25 to 100%, preferably 60 to 95%.
II. Production of Bowl-Shaped Polymer
The bowl-shaped polymer particle of the present invention has a wall which
is usually of a multi-layer structure, and is made by a multi-stage
polymerization process comprising the following steps (1) to (7).
(1) A monomer mixture comprising 30 to 65% by weight of an acidic
group-containing monomer and 70 to 35% by weight of a monomer
copolymerizable therewith is copolymerized to prepare a nucleus polymer
particle.
(2) A monomer mixture comprising 10 to 35% by weight of an acidic
group-containing monomer and 90 to 65% by weight of a monomer
copolymerizable therewith is copolymerized in the presence of the nucleus
polymer particle, prepared in step (1), to prepare a core polymer particle
substantially surrounding the nucleus polymer particle, wherein the
content of the acidic group-containing monomer units in the core polymer
particle is equal to or smaller than the content of the acid
group-containing monomer units in the nucleus polymer particle.
(3) A monomer mixture comprising 1 to 12% by weight of an acidic
group-containing monomer and 99 to 88% by weight of a monomer
copolymerizable therewith is copolymerized in the presence of the
nucleus/core polymer particle, prepared in step (2), to form at least one
intermediate polymer layer which substantially surrounds the nucleus/core
polymer particle;
(4) An aromatic vinyl monomer is polymerized alone or a monomer mixture
comprising up to 0.5% by weight of an acidic group-containing monomer and
at least 99.5% by weight of an aromatic vinyl monomer copolymerizable
therewith is copolymerized, in the presence of the polymer particle which
is prepared in step (3) and comprising the intermediate polymer layer
substantially surrounding the nucleus/core polymer particle, to prepare an
outer polymer layer which substantially surrounds the polymer particle
prepared in step (3).
(5) A base is added to an aqueous dispersion of the polymer particle, which
is prepared in step (4) and has a multi-layer structure comprising at
least four layers, to adjust the pH value of the aqueous polymer
dispersion to at least 7.
(6) If desired, the pH value of the aqueous polymer particle dispersion is
adjusted below 7, and then, an aromatic vinyl monomer alone or a monomer
mixture comprising at least 90% by weight of an aromatic vinyl monomer and
not larger than 10% by weight of a monomer copolymerizable therewith is
added, to polymerize the aromatic vinyl monomer or copolymerize the
monomer mixture in the presence of the polymer particle with the
multi-layer structure, prepared in step (5), to form an outermost polymer
layer surrounding the outer polymer layer prepared in step (4).
(7) The thus-prepared aqueous polymer dispersion is dried.
An assembly of multiplicity of bowl-shaped polymer particles with wall
having a multi-layer structure is made by the above-mentioned process. The
assembly of the bowl-shaped polymer particles usually contains smaller
than about 20% by weight of polymer particles having other shapes, which
do not satisfy the above-mentioned definition for the bowl-shaped polymer
particle.
The above-mentioned multi-stage polymerization process for producing the
bowl-shaped polymer particles with wall having a multi-layer structure
will now be described in detail.
Preparation of Nucleus Polymer Particle [Step (1)]
The nucleus polymer particle is prepared by copolymerizing a monomer
mixture comprising 30 to 65% by weight, preferably 30 to 55% by weight, of
an acidic group-containing monomer and 70 to 35% by weight, preferably 70
to 45% by weight, of a monomer copolymerizable therewith. If the amount of
an acidic group-containing monomer is too small, a base does not easily
penetrate the polymer particle in the base-treating step (5) and it is
difficult to form a void within the polymer particle, and consequently,
the bowl-shaped polymer particle is difficult to prepare. In contrast, if
the amount of an acidic group-containing monomer is too large, the nucleus
polymer particle has a tendency of shifting from the center to the outside
of the intermediate polymer layer and the outer polymer layer, and the
polymerization stability becomes poor.
The acidic group-containing monomer used herein means a monomer having a
functional group exhibiting acidity, and, as specific examples thereof,
there can be mentioned ethylenically unsaturated carboxylic acids such as
acrylic acid, methacrylic acid, crotonic acid, cinnamic acid, itaconic
acid, fumaric acid, maleic acid and butenetricarboxylic acid; monoalkyl
esters of unsaturated dicarboxylic acids such as monoethyl itaconate,
monobutyl fumarate and monobutyl maleate; and sulfonic acid
group-containing monomers such as styrenesulfonic acid. These acidic
group-containing monomers may be used either alone or as a combination of
two or more thereof. With a monomer having a strong acidity, the acidic
group tends to shift to the outside of the polymer particle, and it
becomes difficult that the core polymer and the intermediate polymer layer
cover the nucleus polymer particle, and consequently, the bowl-shaped
polymer particle becomes difficult to prepare.
Of the acidic group-containing monomers, monocarboxylic acids and
monoesters of dicarboxylic acids are preferable. Methacrylic acid is most
preferable.
As specific examples of the monomer copolymerizable with the acidic
group-containing monomers, there can be mentioned aromatic vinyl monomers
such as styrene, .alpha.-methylstyrene, p-methylstyrene and halogenated
styrenes; acrylic acid esters and methacrylic acid esters such as methyl
acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl
acrylate, butyl methacrylate, 2-ethylhexyl acrylate, 2-ethylexyl
methacrylate, lauryl acrylate, lauryl methacrylate, glycidyl acrylate,
glycidyl methacrylate, 2-hydroxyethyl acrylate and 2-hydroxyethyl
methacrylate; acrylamide, methacrylamide and their derivatives such as
acrylamide, methacrylamide, N-methylolacrylamide,
N-methylol-methacrylamide, N-butoxymethylacrylamide and
N-butoxymethyl-methacrylamide; diene monomers such as butadiene and
isoprene; a vinyl ester of carboxylic acids such as vinyl acetate; vinyl
halides such as vinyl chloride; vinylidene halides such as vinylidene
chloride; and vinylpyridine. Of these, acrylic acid esters and methacrylic
acid esters are preferable. These monomers may be used either alone or as
a combination of two or more thereof.
If desired, a cross-linking monomers such as divinylbenzene, diallyl
phthalate, allyl acrylate, allyl methacrylate, ethylene glycol diacrylate
and ethylene glycol dimethacrylate may be used. However, it should be
noted that, when a cross-linking monomer is sued in a salient amount, the
bowl-shaped polymer particle is difficult to prepare, and thus, the amount
thereof should be within the range wherein stable voids can be formed.
As the polymerization procedure for preparing the nucleus polymer particle,
an emulsion polymerization procedure is usually employed. But, phase
inversion may be employed for inverting a polymer prepared by other
polymerization procedure into a latex. The polymerization may be carried
out by a batchwise, semi-continuous or continuous polymerization process.
The polymerization temperature may be either high or low. The
polymerization pressure and time are not particularly limited, and
conventional conditions may be employed. Known auxiliaries may be used,
which include emulsifiers, polymerization initiators, chelating agents,
electrolytes, molecular weight modifiers and surface active agents. A seed
is preferably used for the preparation of the nucleus polymer particle for
easily controlling the size of the nucleus polymer particle. The
composition of seed is not particularly limited. The conversion of
monomers in the polymerization is usually at least 90%, and preferably at
least 97%. The composition of the resulting copolymer is approximately the
same as the composition of the monomer mixture used.
Preparation of Core Polymer Particle [Step (2)]
The core polymer particle is obtained by copolymerizing a monomer mixture
comprising 10 to 35% by weight, preferably 15 to 35% by weight, of an
acidic group-containing monomer and 90 to 65% by weight, preferably 85 to
65% by weight, of a monomer copolymerizable therewith in the presence of
the nucleus polymer particle, prepared in step (1). The content of the
acidic group-containing monomer units in the core polymer particle should
be equal to or smaller than the content of the acid group-containing
monomer units in the nucleus polymer particle. If the content of the
acidic group-containing monomer units is too small, a base does not easily
penetrate the polymer particle in the base-treating step (5) and the
bowl-shaped polymer particle is difficult to prepare. In contrast, if the
content of the acidic monomer units is too large, the core polymer
particle has a tendency of shifting from the center to the outside of the
intermediate polymer layer and the outer polymer layer, the stability of
as-produced polymer becomes poor and the bowl-shaped polymer particle is
difficult to prepare.
As specific examples of the acidic group-containing monomer and the monomer
copolymerizable therewith, there can be mentioned those which are recited
as for the preparation of The nucleus polymer particle. These monomers may
be used either alone or as a combination of two or more thereof.
If desired, a minor amount of a cross-linking monomer can be used. As the
procedure for polymerization, an emulsion polymerization procedure is
usually employed, as the polymerization for the nucleus polymer particle.
The polymerization process (batchwise, semi-continuous or continuous),
polymerization conditions and auxiliaries may be selected from those which
are known, similarly to the case of the preparation of the nucleus polymer
particle.
The monomer mixture for forming the core polymer particle is used in an
amount such that the amount of the nucleus polymer particle is in the
range of 1 to 25 parts by weight, preferably 3 to 20 parts by weight,
based on 100 parts by weight of the core polymer particle. If the relative
amount of the nucleus polymer particle is too large, the bowl-shaped
polymer particle is difficult to prepare. In contrast, the relative amount
of the nucleus polymer particle is too small, the polymerization stability
is reduced.
The thus-prepared nucleus/core polymer particle has a particle diameter in
the range of preferably 150 to 500 nm, and more preferably 200 to 400 nm.
When the polymer particle size is smaller, the bowl-shaped polymer
particle is difficult to prepare. When the polymer particle size is
larger, the polymerization stability becomes poor.
Formation of Intermediate Polymer Layer [Step (3)]
The intermediate polymer layer is formed by copolymerizing a monomer
mixture comprising 1 to 12% by weight, preferably 2 to 10% by weight, more
preferably 3 to 9% by weight, of an acidic group-containing monomer and 99
to 88% by weight, preferably 98 to 90% by weight, more preferably 97 to
91% by weight, of a monomer copolymerizable therewith in the presence of
the nucleus/core polymer particle. If the amount of the acidic
group-containing monomer is too small, a base does not easily penetrate
the polymer particle in the base-treating step (5) and the bowl-shaped
polymer particle is difficult to prepare. In contrast, if the amount of
the acidic monomer units is too large, the polymerization stability
becomes poor. The ratio of the core polymer particle to the intermediate
polymer layer is usually in the range of 5/95 to 60/40 by weight, and
preferably 10/90 to 50/50 by weight. If this ratio is too large (i.e., the
relative amount of the core polymer particle is too large), the
bowl-shaped polymer particle is difficult to prepare.
As specific examples of the acidic group-containing monomer and the monomer
copolymerizable therewith, there can be mentioned those which are recited
as for the preparation of the nucleus polymer particle. These monomers may
be used either alone or as a combination of two or more thereof.
If desired, a minor amount of a cross-linking monomer can be used. As the
procedure for polymerization, an emulsion polymerization procedure is
usually employed, as the polymerization for the nucleus polymer particle.
The polymerization process (batchwise, semi-continuous or continuous),
polymerization conditions and auxiliaries may be selected from those which
are known, similarly to the case of the preparation of the nucleus polymer
particle.
The polymer particle, prepared in step (3) and comprising the intermediate
polymer layer substantially surrounding the nucleus/core polymer particle,
has a particle diameter in the range of preferably 300 to 900 nm, and more
preferably 350 to 800 nm. If the particle diameter is smaller, the
opacifying effect and the heat insulating effect are poor. In contrast, if
the particle diameter is larger, the polymerization stability becomes
poor.
The ratio by weight of the nucleus/core polymer particle to the polymer
particle, prepared in step (3) and comprising the intermediate polymer
layer substantially surrounding the nucleus/core polymer particle, is
usually at least 0.05, preferably at least 0.1 and more preferably at
least 0.17. As this weight ratio increases, the bowl-shaped polymer
particle becomes easy to prepare.
Preparation of Outer polymer Layer [Step (4)]
The outer polymer layer is formed on the periphery of the intermediate
polymer layer formed in step (3), and substantially surrounds the
nucleus/core/intermediate layer polymer particle. The outer polymer layer
is formed by polymerizing an aromatic vinyl monomer alone or
copolymerizing a monomer mixture comprising up to 0.5% by weight of an
acidic group-containing monomer and at least 99.5% by weight of an
aromatic vinyl monomer copolymerizable therewith in the presence of the
polymer particle comprising the intermediate polymer layer, prepared in
step (3).
The monomer used for the formation of the outer polymer layer is an
aromatic vinyl monomer alone or a monomer mixture comprising at least
99.5% by weight of an aromatic vinyl monomer. Preferably an aromatic vinyl
monomer is used alone. If the amount of an aromatic vinyl monomer is
smaller than 99.5% by weight, the opacifying effect and heat insulation
effect of the bowl-shaped polymer particle become poor.
As specific examples of the aromatic vinyl monomer, there can be mentioned
styrene, .alpha.-methylstyrene, p-methylstyrene and halogenated styrenes.
Of these, styrene is most preferable.
As specific examples of the acidic group-containing monomer to be
copolymerized with the aromatic vinyl monomer, there can be mentioned
those which are recited as for the preparation of the nucleus polymer
particle. If the amount of the acidic group-containing monomer exceeds
0.5% by weight, the bowl-shaped polymer particle becomes to prepare.
The ratio by weight of the polymer particle, prepared in step (3) and
comprising the intermediate polymer layer substantially surrounding the
nucleus/core polymer particle, to the polymer particle, prepared in step
(4) and having the outer polymer layer substantially surrounding the
polymer particle prepared in step (3), is preferably in the range of 0.4
to 0.6. If the ratio is smaller than this range, the bowl-shaped polymer
particle is difficult to prepare.
The ratio by weight of the nucleus/core polymer particle to the polymer
particle, prepared in step (4) and having the outer polymer layer, is
usually at least 0.02, preferably at least 0.04 and more preferably at
least 0.07. The bowl-shaped polymer particle can be easily made with an
increase of this ratio. In contrast, if this ratio is too small, the core
polymer has a tendency of shifting to the outside of the outer polymer
when polymerization is carried out for formation of the outer polymer
layer or at the base-treating step, and thus, void is difficult to occur
within the polymer particle at the base-treating step, and the bowl-shaped
polymer particle becomes difficult to prepare.
As the polymerization procedure for forming the outer polymer layer,
emulsion polymerization is usually employed, which is similar to
polymerization for the nucleus polymer particle, the core polymer particle
and the intermediate polymer particle. The polymerization process
(batchwise, semi-continuous or continuous), polymerization conditions and
auxiliaries may be similarly selected from those which are known.
Treatment with Base [Step (5)]
A base is added to an aqueous dispersion, such as a latex, of the polymer
particle, prepared in step (4) and having a multi-layer structure
comprising the nucleus polymer particle, the core polymer particle, the
intermediate polymer layer and the outer polymer layer, to adjust the pH
value of the aqueous polymer dispersion to at least 7, whereby a void
(i.e., a vacant space) is formed within the polymer particle. At this
stage wherein a base is added thereto, the void is filled with an aqueous
medium which constitutes the aqueous dispersion of the polymer particle.
As specific examples of the base, there can be mentioned alkali metal
hydroxides such as sodium hydroxide, potassium hydroxide and lithium
hydroxide; alkaline earth metal hydroxides such as calcium hydroxide and
magnesium hydroxide; alkali metal carbonates and bicarbonates such as
sodium carbonate and potassium bicarbonate; and ammonium carbonate and
ammonium bicarbonate. Of these, alkali metal hydroxides are preferable.
The amount of the base is such that at least part of the acid groups
within the polymer particle is neutralized whereby the pH value of the
aqueous polymer particle dispersion becomes at least 7.
To neutralize the acidic groups within the polymer particle by the addition
of a base to the aqueous polymer particle dispersion, a time is required
for which a base is diffused inside the polymer particle. Therefore, after
the addition of a base, stirring is preferably carried out for a
sufficient time. The temperature at which the base treatment is carried
out is preferably such that the polymer particle is softened thoroughly.
After the addition of a base, the treatment is carried out usually for a
period of about 15 to 120 minutes.
It is possible that the stability of the aqueous dispersion is reduced by
the addition of a base. To prevent the reduction of stability, an anionic
or nonionic surface active agent may be added before the addition of a
base. The surface active agent may be used either alone or in combination.
JP-A H3-26724 discloses a process for the preparation of hollow polymer
particles including the step of treating with a base in the presence of an
organic solvent. In contrast, it is rather preferable in the present
invention that the base treatment is carried out in the absence of an
organic solvent. If the base treatment is carried out in the presence of
an organic solvent, especially a non-polymerizable aliphatic hydrocarbon,
the finally produced polymer particles have a flat shape rather than a
bowl shape.
The base treatment can be carried out in the presence of a polymerizable
monomer. Usually a monomer not containing an acidic group is used. The
amount of the polymerizable monomer is usually in the range of 1 to 20
parts by weight, preferably 2 to 10 parts by weight, based on 100 parts by
weight of the total monomers used for the production of polymer particles.
Acid Treatment [Step (5')]
The polymer latex, thus-treated with a base, may be treated with an acid to
lower the pH value below 7, if desired. By the acid treatment, the
particle diameter and the void diameter can be made large.
The acid used for the optional acid-treatment is not particularly limited,
and includes, for example, inorganic acids such as hydrochloric acid and
sulfuric acid; and organic acids such as acetic acid and malonic acid. An
acidic group-containing monomer may also be used as the acid for the acid
treatment. The amount of the acidic group-containing monomer is usually in
the range of 0.01 to 40 parts by weight, preferably 0.05 to 20 parts by
weight and more preferably 0.2 to 10 parts by weight, based on 100 parts
by weight of the total monomers used for the preparation of the
nucleus/core/intermediate layer/outer layer polymer particles.
In the acid treatment, the acidic group-containing monomer may be used in
combination with a monomer copolymerizable therewith. The amount of the
copolymerizable monomer is usually in the range of 0.1 to 20 parts by
weight, preferably 0.5 to 10 parts by weight, based on 100 parts by weight
of the monomer or monomer mixture for the outermost layer. As examples of
the copolymerizable monomer, there can be mentioned those which are
recited as for the preparation of the core polymer particle.
The temperature, time and other conditions for the acid treatment are
approximately the same as those for the base treatment. It is possible
that the stability of the aqueous dispersion is reduced by the addition of
an acid. If desired, to prevent the reduction of stability, an anionic or
nonionic surface active agent may be added prior to the addition of an
acid. The surface active agent may be used either alone or in combination.
Formation of Outermost Polymer Layer [Step (6)]
After the base treatment, or the acid treatment, if desired, an aromatic
vinyl monomer or a monomer mixture comprising at least 90% by weight of an
aromatic vinyl monomer and not larger than 10% by weight of a monomer
copolymerizable therewith is added to polymerize the aromatic vinyl
monomer or copolymerize the monomer mixture in the presence of the polymer
particle with the multi-layer structure, to form an outermost polymer
layer surrounding the outer polymer layer of the polymer particle. By the
polymerization of an aromatic vinyl monomer or a monomer mixture
predominantly comprised of an aromatic vinyl monomer, an outermost polymer
layer having a high glass transition temperature (Tg) is formed, and thus,
the opacifying effect and heat-insulating effect of the hollow polymer
particle are enhanced and melt-adhesion of polymer articles is prevented.
As mentioned above, the monomer or monomer mixture for forming the
outermost layer may be added to the aqueous polymer particle dispersion in
the base treatment step to promote the diffusion of a base within the
polymer particle. As the monomer used for copolymerization in combination
with an aromatic vinyl monomer, an acidic group-containing monomer is
preferably used. The acidic group-containing monomer can be added to the
aqueous polymer particle dispersion for the purpose of treating the
polymer particle with an acid in the acid treatment step, as explained as
for the acid treatment step. When an acidic group-containing monomer is
used in combination with an aromatic vinyl monomer, vacant space ratio of
the hollow polymer particle is enhanced and the bowl-shaped polymer
particle can easily be produced.
If the content of an aromatic vinyl monomer in the monomer mixture for
forming the outermost polymer layer is smaller than 90% by weight, the
opacifying effect and the heat-insulating effect are reduced and the
polymer particles are liable to be melt-adhered.
The ratio by weight of the polymer particle, prepared in step (4) and
having the outer polymer layer substantially surrounding the polymer
particle prepared in step (3), to the outermost polymer layer is usually
in the range of 100/10 to 50/100 and preferably 100/25 to 100/100.
If desired, a minor proportion of a cross-linking monomer may be used as a
part of the monomer mixture for forming the outermost polymer layer.
As the polymerization procedure for forming the outermost polymer layer, an
emulsion polymerization procedure is usually employed, which is similar to
polymerization for the other polymer layers. The polymerization process
(batchwise, semi-continuous or continuous), polymerization conditions and
auxiliaries may be similarly selected from those which are known.
Drying [Step (7)]
By the formation of the outermost polymer layer, hollow polymer particles
having at least one void are obtained. At the stage of completion of
polymerization, the void within the hollow polymer particles is filled
with the aqueous liquid. When the hollow polymer particles are dried to
remove the aqueous liquid, bowl-shaped polymer particles can be obtained.
It is presumed that, with a progress of drying and a removal of the aqueous
liquid within the polymer particle, the outer shell forming the hollow
polymer particle is caved in an inwardly folded manner whereby the entire
polymer particle becomes bowl-shaped. More specifically, with a progress
of drying, the degree of caving of the outer shell progresses and the
volume of vacant space within the polymer particle is gradually reduced,
and finally, the volume of vacant space reaches zero or in the vicinity
thereof whereby the entire polymer particle becomes bowl-shaped. The
drying can be conducted either by directly drying the aqueous dispersion
(i.e., latex), for example, by spray drying, or by coagulating the aqueous
dispersion, for example, by a conventional coagulating procedure, to
separate the solid hollow polymer particle, followed by drying the
separated polymer particle. By drying relatively rapidly, a solid
bowl-shaped polymer particle with no vacant space within the particle can
be obtained. By the spray-drying, a solid bowl-shaped polymer particle
with no vacant space therein can be directly obtained.
III. Use of Bowl-Shaped Polymer Particle, Thermal Recording Material
The bowl-shaped polymer particle of the present invention is used as a
heat-insulating material, an organic pigment, an opacifying agent and
others, and is incorporated in a heat-insulating intermediate
layer-forming material, an aqueous paint, a coating for paper, and others.
A typical example of the use is a material for forming a heat-insulating
intermediate layer between a support and a thermal recording layer in a
thermal recording material having the thermal recording layer formed on
the support. Thus, the thermal recording material comprises a support, a
heat-insulating intermediate layer comprising the bowl-shaped polymer
particle of the invention, and a thermal recording layer.
The support is generally selected from sheet-form articles such as paper,
synthetic paper, nonwoven fabric, and plastic film or sheet, but articles
other than sheet-form articles can be used.
To form on a support the heat-insulating intermediate layer comprising the
bowl-shaped polymer particle of the invention, the bowl-shaped polymer
particle is dispersed in water together with a binder, the thus-prepared
aqueous dispersion is coated on a support, and the coating is dried. The
coating procedure is not particularly limited, and conventional coating
procedures using, for example, an off-machine coating apparatus or an
on-machine coating apparatus, provided with a coater such as an air knife
coater, a rod blade coater, a billblade coater or a roll coater, can be
adopted.
As specific examples of the binder, there can be mentioned polyvinyl
alcohol and its derivatives such as partially or completely saponified
polyvinyl alcohol, carboxyl-modified polyvinyl alcohol, amide-modified
polyvinyl alcohol, sulfonic acid-modified polyvinyl alcohol,
butyral-modified polyvinyl alcohol, and other modified polyvinyl alcohols;
starch and its derivatives; cellulose derivatives such as
methoxycellulose, hydroxyethyl cellulose, carboxymethyl cellulose, methyl
cellulose, and ethyl cellulose; water-soluble polymeric materials such as
sodium polyacrylate, polyvinyl pyrrolidone, acrylamide/acrylic acid ester
copolymers, acrylamide/acrylic acid ester/methacrylic acid terpolymers, an
alkali metal salt of styrene/maleic anhydride copolymer, an alkali metal
salt of isobutylene/maleic anhydride copolymer, polyacrylamide, sodium
alginate, gelatine and casein; latexes of polymers such as
acrylonitrile/butadiene copolymer, styrene/butadiene copolymer, and
styrene/butadiene/acrylate copolymer; and aqueous emulsions of polymeric
materials such as vinyl chloride resin, vinyl acetate resin, vinyl
acetate/acrylic acid copolymer, styrene/acrylic acid ester copolymer,
acrylic acid ester resin, polyvinyl butyral, polyamide resin, and
polyurethane resin.
The amount of the bowl-shaped polymer particle in the coating for forming
the intermediate lever is in the range of 5 to 95% by weight, preferably
60 to 85% by weight, based on the total solid content of the polymer
particle and the binder. The amount of the binder in the coating for
forming the intermediate layer is in the range of 95 to 5% by weight,
preferably 40 to 15% by weight, based on the total solid content of the
polymer particle and the binder. The amount of the coating liquid applied
onto the support is in the range of 1 to 30 g/m.sup.2, preferably 3 to 20
g/cm.sup.2, as expressed as the amount of the bowl-shaped polymer
particle.
If desired, auxiliaries conventionally added to thermal recording material
can be incorporated together with the bowl-shaped polymer particle and the
binder. The auxiliaries include, for example, fillers, dispersants,
defoaming agents, fluorescent dyes, coloring dyes, inorganic pigments,
organic pigments, wax and other lubricants, ultraviolet absorbers such as
benzophenone-type and triazole-type ultraviolet absorbers, sensitizer, and
electrically conductive substances.
If desired, an additional layer comprising a pigment and other auxiliaries,
and a binder, can be formed, in addition to the intermediate layer
comprising the bowl-shaped polymer particle, between the intermediate
layer and the support, and/or between the intermediate layer and the
thermal recording layer.
As the pigment, known organic and inorganic pigments can be used. As
specific examples of the inorganic pigment, there can be mentioned
alumina, magnesium hydroxide, calcium hydroxide, magnesium carbonate, zinc
oxide, barium sulfate, silica, calcium carbonate, kaoline, calcined
kaoline, diatomaceous earth, talc, titanium oxide and aluminum hydroxide.
As specific examples of the organic pigment, there can be mentioned
urea-formaldehyde resin, styrene/methacrylic acid copolymer, polystyrene
resin, and amino resin filler. Of these, those which have an oil
absorption of about 30 to 200 cc/100 g is preferable.
When a thermal recording layer is formed on the intermediate layer, if
desired, calendering can be conducted on the intermediate layer to render
the surface smooth, and then, a coating for forming the thermal recording
layer is applied onto the smooth intermediate layer.
The thermal recording layer comprises a combination of a color producing
reagent with a developer. The combination of a color producing reagent
with a developer is not particularly limited, provided that, when the
color producing reagent and the developer are heated and placed in contact
with each other, a color reaction occurs. The combination of a color
producing reagent with a developer includes, for example, leuco dye-type
color developing material comprising a combination of a colorless or
light-colored basic dye with an inorganic or organic acidic substance, a
chelate-type color developing material comprising a combination of a metal
salt of a higher fatty acid such as ferric stearate with stearyl gallate,
a pigment-type color developing material comprising an imino compound with
an isocyanate compound, and a color developing material comprising a
diazonium compound, coupler and a basic substance. Of these, color
developing materials, leuco dye-type color developing material comprising
a combination of a colorless or light-colored basic dye with and acidic
substance is preferable.
As specific examples of the colorless or light-colored basic dye, there can
be mentioned triphenylmethane-type leuco dyes such as
3,3-bis(p-dimethylaminophenyl)-6-dimethylamino-pthalide; fluoran-type
leuco dyes such as 3-diethylamino-6-methyl-7-anilinofluoran,
3-(N-ethyl-p-toluidino)-6-methyl-7-anilinofluoran,
3-(N-ethyl-p-isoamylamino)-6-methyl-7-anilinofluoran,
3-diethylamino-6-methyl-7-(o,p-dimethylanilino)fluoran,
3-pyrrolidino-6-methyl-7-anilinofluoran,
3-piperidino-6-methyl-7-anilinofluoran,
3-(N-cyclohexyl)-N-methylamino)-6-methyl-7-anilinofluoran,
3-diethylamino-7-(m-trifluoromethylanilino)fluoran,
3-N-n-dibutylamino-6-methyl-7-anilinofluoran,
3-N-n-dibutylamino-7-(O-CHLOROANILINO)FLUORAN,
3-(N-ethyl-N-tetrahydrofurfurylamino)-6-methyl-7-anilinofluoran,
3-dibutylamino-6-methyl-7-(o,p-dimethylanilino)fluoran,
3-(N-methyl-N-propylamino-6-methyl-7-anilinofluoran,
3-diethylamino-6-chloro-7-anilinofluoran,
3-dibutylamino-7-(o-chloroanilino)fluoran,
3-diethylamino-7-(o-chloroanilino)fluoran,
3-diethylamino-6-methyl-7-(m-methylanilino)fluoran,
3-n-dibutylamino-6-methyl-7-(m-methylanilino)fluoran,
3-diethylamino-6-methyl-chlorofluoran, 3-diethylamino-6-methyl-fluoran,
3-cyclohexyl-amino-6-chlorofluoran, 3-diethylamino-benzo[a]-fluoran,
3-n-dipentylamino-6-methyl-7-anilinofluoran,
2-(4-oxo-hexyl-3-dimethylamino-6-methyl-7-anilinofluoran,
2-(4-oxo-hexyl-3-diethylamino-6-methyl-7-anilinofluoran,
2-(4-oxo-hexyl-3-dipropylamino-6-methyl-7-anilinofluoran,
2-methyl-6-p-(p-dimethylaminophenyl)aminoanilinofluoran,
2-chloro-3-methyl-6-p-(p-phenylaminophenyl)aminoanilinofluoran,
2-methoxy-6-p-(p-dimethylaminophenyl)aminoanilinofluoran,
2-chloro-6-p-(p-dimethylaminophenyl)aminoanilinofluoran,
2-nitro-6-p-(p-diethylaminophenyl)aminoanilinofluoran,
2-amino-6-p-(p-diethylaminophenyl)aminoanilinofluoran,
2-diethylamino-6-p-(p-diethylaminophenyl)aminoanilinofluoran,
2-phenyl-6methyl-6-p-(p-phenylaminophenyl)aminoanilinofluoran,
2benzyl-6-p-(p-phenylaminophenyl)aminoanilinofluoran,
2-hydroxy-6-p-(p-phenylaminophenyl)aminoanilinofluoran,
3-methyl-6-p-(p-dimethylaminophenyl)aminoanilinofluoran,
3-diethylamino-6-p-(p-diethylaminophenyl)aminoanilinofluoran and
3-diethylamino-6-p-(p-dibutylaminophenyl)aminoanilinofluoran;
fluorene-type leuco dyes such as
3,6,6'-tris(dimethylamino)spiro[fluorene-9,3'-phthalide] and
3,6,6'-tris(diethylamino)spiro[fluorene-9,3'-phthalide]; divinyl-type
leuco dyes such as
3,3-bis-[2-(p-dimethylaminophenyl)-2-(p-methyoxyphenyl)ethenyl]-4,5,6,7-te
trabromophthalide,
3,3-bis-[2-(p-dimethylaminophenyl)-2-(p-methoxyphenyl)-ethenyl]-4,5,6,7-te
trachlorophthalide,
3,3-bis-[1,1-bis(4-pyrrolidinophenyl)-ethylen-2-yl]-4,5,6,7-tetrabromo-pht
halide and
3,3-bis-[1-(4-methoxyphenyl)-1-(4-pyrrodinophenyl)ethylen-2-yl]-4,5,6,7-te
trachlorophthalide; and
1,1-bis-[2',2',2",2"-tetrakis-(p-dimethylaminophenyl)-ethenyl]-2,2-dinitri
loethane,
1,1-bis-[-2',2',2",2"-tetrakis-(p-dimethylaminophenyl)-ethenyl]-2-.beta.-n
aphthoylethane,
1,1-bis-[2',2',2",2"-tetrakis-(p-dimethylaminophenyl)-ethenyl]-2,2-diacety
lethane and
bis-[2,2,2',2'-tetrakis-(p-dimethylaminophenyl)-ethenyl]-methylmalonic
acid dimethyl ester. These dyes may be used either alone or as a mixture
of at least two thereof.
As specific examples of the developer used in combination with the
above-recited leuco dyes, there can be mentioned bisphenols A such as
4,4'-isopropyridenediphenol (i.e., bisphenol A),
4,4'-cyclohexylidenediphenol, p,p'-(1-methylnormalhexylidene)diphenol and
1,7-di(hydroxyphenylthio)-3,5-dioxaheptane; 4-hydroxybenzoic acid esters
such as benzyl 4-hydroxybenzoate, ethyl 4-hydroxybenzoate, propyl
4-hydroxybenzoate, isopropyl 4-hydroxybenzoate, butyl 4-hydroxybenzoate,
isobutyl 4-hydroxybenzoate and methylbenzyl 4-hydroxybenzoate;
4-hydroxyphthalic acid diesters such as dimethyl 4-hydroxyphthalate,
diisopropyl 4-hydroxyphthalate, dibenzyl 4-hydroxyphthalate and dihexyl
4-hydroxyphthalate; phthalic acid monoesters such as monobenzyl phthalate,
monocyclohexyl phthalate, monophenyl phthalate, monomethylphenyl
phthalate, monoethylphenyl phthalate, monopropylbenzyl phthalate,
monohalogenobenzyl phthalate and monoethoxybenzyl phthalate;
bis-(hydroxyphenyl)sulfides such as
bis-(4-hydroxy-3-tert-butyl-6-methylphenyl)sulfide,
bis-(4-hydroxy-2,5-dimethylphenyl)sulfide,
bis-(4-hydroxy-2-methyl-5-ethylphenyl)sulfide,
bis-(4-hydroxy-2-methyl-5-isopropylphenyl)sulfide,
bis-(4-hydroxy-2,3-dimethylphenyl)sulfide,
bis-(4-hydroxy-2,5-dimethylphenyl)sulfide,
bis-(4-hydroxy-2,5-diisopropylphenyl)sulfide,
bis-(4-hydroxy-2,3,6-trimethylphenyl)sulfide,
bis-(2,4,5-trihydroxyphenyl)sulfide,
bis-(4-hydroxy-2cyclohexyl-5-methylphenyl)sulfide,
bis-(2,3,4-trihydroxyphenyl)sulfide,
bis-(4,5-dihydroxy-2-tert-butylphenyl)sulfide and
bis-(4-hydroxy-2-tert-octyl-5-methylphenyl)sulfide; 4-hydroxyphenyl aryl
sulfones such as 4-hydroxy-4'-iospropoxydiphenyl sulfone,
4-hydroxy-4'-propoxydiphenyl sulfone, 4-hydroxy-4'-n-butyloxydiphenyl
sulfone and 4-hydroxy-4'-n-propoxydiphenyl sulfone; 4-hydroxyphenylaryl
sulfonates such as 4-hydroxyphenylbenzene sulfonate,
4-hydroxyphenyl-p-tolyl sulfonate, 4-hydroxyphenylmethylene sulfonate,
4-hydroxyphenyl-p-chlorobenzene sulfonate,
4-hydroxyphenyl-p-tert-butylbenzene sulfonate,
4-hydroxyphenyl-p-isopropoxybenzene sulfonate,
4-hydroxyphenyl-1'-naphthalene sulfonate and
4-hydroxyphenyl-2'-naphthalene sulfonate;
1,3-di[2-hydroxyphenyl)-2-propyl]-benzenes such as
1,3-di[2-(4-hydroxyphenyl)-2-propyl]-benzene,
1,3-di[2-(4-hydroxy)-3-alkylphenyl)-2-propyl]-benzene,
1,3-di[2-(2,4-dihydroxyphenyl)-2-propyl]-benzene and
1,3-di[2-(2-hydroxy-5-methylphenyl)-2-propyl]-benzene; resorcinols such as
1,3-dihydroxy-6-(.alpha.,.alpha.-dimethylbenzyl)-benzene;
4-hydroxybenzoyloxybenzoic acid esters such as benzyl
4-hydroxybenzoyloxybenzoate, methyl 4-hydroxybenzoyloxybenzoate, ethyl
4-hydroxybenzoyloxybenzoate, propyl 4-hydroxybenzoyloxybenzoate, butyl
4-hydroxybenzoyloxybenzoate, isopropyl 4-hydroxybenzoyloxybenzoate,
tert-butyl 4-hydroxybenzoyloxybenzoate, hexyl 4-hydroxybenzoyloxybenzoate,
octyl 4-hydroxybenzoyloxybenzoate, nonyl 4-hydroxybenzoyloxybenzoate,
cyclohexyl 4-hydroxybenzoyloxybenzoate, .beta.-phenethyl
4-hydroxybenzoyloxybenzoate, phenyl 4-hydroxybenzoyloxybenzoate,
.alpha.-naphthyl 4-hydroxybenzoyloxybenzoate, .beta.-naphthyl
4-hydroxybenzoyloxybenzoate and sec-butyl 4-hydroxybenzoyloxybenzoate;
bisphenol sulfones such as bis-(3-1-butyl-4-hydroxy-6-methylphenyl)
sulfone, bis-(3-ethyl-4-hydroxyphenyl) sulfone,
bis-(3-propyl-4-hydroxyphenyl) sulfone, bis-(3-methyl-4-hydroxyphenyl)
sulfone, bis-(2-isopropyl-4-hydroxyphenyl) sulfone,
bis-(2-ethyl-4-hydroxyphenyl) sulfone, bis-(3-chloro-4-hydroxyphenyl)
sulfone, bis-(2,3-dimethyl-4-hydroxyphenyl) sulfone,
bis-(2,5-dimethyl-4-hydroxyphenyl) sulfone,
bis-(3-methoxy-4-hydroxyphenyl) sulfone,
4-hydroxyphenyl)-2'-ethyl-4'-hydroxyphenyl sulfone,
4-hydroxyphenyl)-2'-isopropyl-4'-hydroxyphenyl sulfone,
4-hydroxyphenyl)-3'-isopropyl-4'-hydroxyphenyl sulfone,
4-hydroxyphenyl)-3'-sec-butyl-4'-hydroxyphenyl sulfone,
3-chloro-4-hydroxyphenyl-3'-isopropyl-4'-hydroxyphenyl sulfone,
2-hydroxy-5-tert-butylphenyl-4'-hydroxyphenyl sulfone,
2-hydroxy-5-tert-amylphenyl-4'-hydroxyphenyl sulfone,
2-hydroxy-5-isopropylphenyl-4'-hydroxyphenyl sulfone,
2-hydroxy-5-tert-octylphenyl-4'-hydroxyphenyl sulfone,
2-hydroxy-5-tert-butylphenyl-3'-chloro-4'-hydroxyphenyl sulfone,
2-hydroxy-5-tert-butylphenyl-3'-methyl-4'-hydroxyphenyl sulfone,
2-hydroxy-5-tert-butylphenyl-3'-isopropyl-4'-hydroxyphenyl sulfone,
2-hydroxy-5-tert-butylphenyl-2'-methyl-4'-hydroxyphenyl sulfone,
4,4'-sulfonyldiphenol, 2,4'-sulfonyldiphenol,
3,3'-dichloro-4,4'-sulfonyldiphenol, 3,3'-dibromo-4,4'-sulfonyldiphenol,
3,3',5,5'-tetrabromo-4,4'-sulfonyldiphenol and
3,3'-diamino-4,4'-sulfonyldiphenol; and
4,4'-bis(p-toluenesulfonylaminocarbonylamino)-diphenylmethane,
p-tert-butylphenol, 2,4-dihydroxy-benzophenone, novolak-type
phenol-formaldehyde resin, 4-hydroxyacetophenone, p-phenylphenol,
benzyl-4-hydroxyphenyl acetate and p-benzyl phenol.
The ratio of the color producing basic dye to the developer is in the range
of 1/1 to 1/8 by weight.
A binder is incorporated in a coating color for forming the thermal
recording layer. The amount of the binder is usually in the range of 5 to
40% by weight, preferably 10 to 25% by weight, based on the total solid
content in the coating color. As the binder, water-soluble high polymers
or aqueous high polymer emulsions are used in a manner similar to that in
the binder for the intermediate layer. As specific examples of the binder,
there can be mentioned those which recited as for the binder for the
intermediate layer.
The thermal recording layer may have incorporated therein a known
sensitizer provided that the object of the present invention is attained.
As specific examples of the sensitizer, there can be mentioned fatty acid
amides such as stearic acid amide and palmitic acid amide, ethylene
bisamide, montan wax, polyethylene wax, dibenzyl terephthalate, benzyl
p-benzyloxy-benzoate, di-p-tolyl carbonate, p-benzylbiphenyl, phenyl
.alpha.-naphthylcarbonate, 1,4-diethoxynaphthalene, phenyl
1-hydroxy-2-naphthoate, 1,2-di-(3-methlphenoxy)ethane, di(p-methyl-benzyl)
oxalate, .beta.-benzyloxynaphthalene, 4-biphenyl p-tolyl ether,
o-xylylene-bis-(phenylether) and 4-(m-methylphenoxy-methyl)biphenyl. These
sensitizers may be used either alone or as a mixture of at least two
thereof.
The thermal recording layer may have incorporated therein a stabilizer such
as p-nitrobenzoic acid metal salt or phthalic acid monobenzyl ester metal
salt, a pigment and a filler. As examples of the pigment and the filler,
there can be mentioned those which are recited as for the intermediate
layer. Further, a release agent such as a fatty acid metal salt, a
lubricant such as wax, an ultraviolet absorber such as a benzophenone- or
triazole-type ultraviolet absorber, a water resistance-imparting agent
such as glyoxal, a dispersant and a defoaming agent may also be
incorporated.
The coating liquid for the thermal recording layer can be applied by a
known procedure in a manner similar to that in the coating liquid for the
intermediate layer.
If desired, to enhance the useful life of the thermal recording material,
an overcoating layer comprising a high polymeric material having
incorporated a pigment therein can be formed on the thermal recording
layer, or a back coating layer comprising a high polymeric material can be
formed on the support.
The bowl-shaped polymer particle and its application for a thermal
recording material will now be specifically described by the following
examples.
In the following examples, parts and % are by weight unless otherwise
specified. The amount of latex was expressed in terms of the weight of
solid content.
EXAMPLE 1
Production of Bowl-Shaped Particle (1)
An emulsion (i) was prepared by mixing under agitation 1 part of a monomer
mixture (a) for preparing a nucleus polymer particle comprising 60% of
methyl methacrylate (MMA), 5% of butyl acrylate (BA) and 35% of
methacrylic acid (MAA), 0.005 part of an emulsifier (DBS) and 0.8 part of
deionized water.
An emulsion (ii) was prepared by mixing under agitation 10 parts of a
monomer mixture (b) for preparing a core polymer particle comprising 70%
of MMA, 10% of BA and 20% of MAA, 0.05 part of an emulsifier (DBS) and 8
parts of deionized water.
An emulsion (iii) was prepared by mixing under agitation 25 parts of a
monomer mixture (c) for forming an intermediate polymer layer comprising
78% of MMA, 16% of BA and 6% of MAA, 0.1 part of an emulsifier (DBS) and
35 parts of deionized water.
An emulsion (iv) for forming an outer polymer layer was prepared by mixing
under agitation 36.9 parts of styrene (ST), 0.3 part of an emulsifier
(DBS) and 16 parts of deionized water.
An emulsion (v) was prepared by mixing under agitation 38.1 parts of a
monomer mixture (d) for forming an outermost polymer layer comprising
96.9% of styrene (ST) and 3.1% of MAA, 0.3 part of an emulsifier (DBS) and
16 parts of deionized water.
A reactor equipped with a stirrer, a reflux condenser, a thermometer and a
dropping funnel was charged with 2.8 parts of deionized water, and 0.04
part (as solid content) of an acrylic polymer seed latex having a particle
diameter of 35 nm and a solid content of 12%, and the content was heated
to 80.degree. C. Then 0.17 part of a 3% aqueous potassium persulfate (KPS)
solution was added through the dropping funnel, and then, the emulsion (i)
was continuously added over a period of 4 hours, and the added monomers
were was further subjected to polymerization for 1 hour to prepare a
nucleus polymer emulsion. The conversion of the monomer mixture (a) was
99%.
Then 28 parts of deionized water and 1.7 parts of a 3% aqueous KPS solution
were added, and the emulsion (ii) was continuously added over a period of
3 hours. After completion of the addition, the added monomers were further
subjected to polymerization for 2 hours to prepare a core polymer. The
conversion of the monomer mixture (b) was 99%.
Then 240 parts of deionized water and 6.7 parts of a 3% aqueous KPS
solution were added, and the emulsion (iii) was continuously added over a
period of 4 hours. After completion of addition, the added monomers were
further subjected to polymerization for 2 hours to form an intermediate
polymer layer. The conversion of the monomer mixture (c) was 99%.
The content was heated to 85.degree. C., and 6.7 parts of a 3% aqueous KPS
solution were added, and the emulsion (iv) was continuously added over a
period of 1.5 hours. After completion of the addition, the added monomers
were further subjected to polymerization for 1 hour to form an outer
polymer layer. The conversion of the monomer mixture (d) was 99%.
To the thus-prepared polymer latex, 9 parts of a 10% aqueous sodium
hydroxide solution were dropwise added through the dropping funnel, and
then, the content was maintained at 85.degree. C. for 30 minutes, to
conduct a base treatment. A portion of the content was sampled, and the pH
value as measured at room temperature was 8.7.
Then 10 parts of a 3% aqueous KPS solution were added, and then the
emulsion (v) was continuously added over a period of 1.5 hours. After
completion of the addition, the added monomers were subjected to
polymerization for 2 hours to prepare a latex containing polymer particles
having the thus-formed outermost polymer layer. The conversion of the
monomer mixture (d) was 99%.
The polymer particle-containing latex was dried by a spray drier having an
inlet temperature of 160.degree. C. and an outlet temperature of
60.degree. C. to prepare polymer particle.
A scanning electron micrograph (SEM) photograph of the polymer particles is
shown in FIG. 2. Observation of the appearance of the polymer particles by
SEM revealed that the polymer particles were bowl-shaped particles such
that a part of each spherical particle having an average maximum diameter
890 nm was concave to form a brim having an average diameter of 690 nm.
EXAMPLE 2
Production of Bowl-Shaped Particle (2)
Polymer particles were produced by the same procedure as that in Example 1
except that the amount of the acrylic polymer see latex was changed to
0.0048 part, the amount of the monomer mixture (a) for preparing a nucleus
polymer particle was changed to 0.12 part, and, as the monomer mixture (b)
for preparing a core polymer particle, 3 parts of a monomer mixture
comprising 60% of MMA, 10% of BA and 30% of MMA were used. Observation of
the appearance of the polymer particles revealed that the thus-obtained
polymer particles were bowl-shaped particles such that a part of each
spherical particle having an average maximum diameter 1,390 nm was concave
to form a brim having an average diameter of 1,100 nm.
EXAMPLE 3
Production and Evaluation of Thermal Recording Material
An intermediate layer-forming coating liquid A was prepared by dispersing
the following ingredients.
Liquid A:
Bowl-shaped polymer particle (A), 100 parts
prepared in Example 1
Styrene/butadiene copolymer latex 11 parts
(solid content: 48%)
10% Aqueous polyvinyl alcohol solution 5 parts
Water 150 parts
Woodfree paper having a basis weight of 50 g/m.sup.2 was coated with liquid
A in an amount of 8 g/m.sup.2 (dry weight) and dried to prepare an
intermediate layer-formed paper.
According to the following recipes, liquid B, liquid C and liquid D were
prepared by pulverizing the respective solid ingredients into an average
particle diameter of not larger than 1 .mu.m by a sand grinder, followed
by dispersing in water.
Liquid B: (Developer dispersion):
4.4'-Isopropylidenediphenol 6.0 parts
10% Aqueous polyvinyl alcohol solution 18.8 parts
water 11.2 parts
Liquid C (Dyestuff dispersion):
3-N-n-Dibutylamino-6-methyl-7-anilinofluoran 2.0 parts
10% Aqueous polyvinyl alcohol solution 4.6 parts
Water 2.6 parts
Liquid D (Sensitizer dispersion):
4-Biphenyl-p-tolylether 4.0 parts
10% Aqueous polyvinyl alcohol solution 5.0 parts
Water 3.0 parts
Liquid B, liquid C and liquid D were mixed together with an aqueous kaoline
clay dispersion according to the following recipe to prepare a coating
solution.
Liquid B 36.0 parts
Liquid C 9.2 parts
Liquid D 12.0 parts
Kaoline clay dispersion (50% aqueous dispersion) 12.0 parts
The thus-prepared coating color was coating in an amount of 6.0 g/m.sup.2
on the above-mentioned intermediate layer-formed wood-free paper. The
coated paper was dried and subjected to calendering to a smoothness degree
of 700-800 sec by using a super-calendar to obtain a thermal recording
sheet.
Dynamic coloring sensitivity, reproducibility of dots and deposition of
tailings on thermal head were evaluated on the thermal recording sheet by
the following methods.
Dynamic Coloring Density
An image was reproduced by using a thermal facsimile UF-1000B supplied by
Matsushita Denso K.K. at a voltage of 14.7 V, a resistance of 360 .OMEGA.,
a pulse width of 0.82 ms and an imposed energy 0.37 mJ/dot. The image
density was measured by using a Macbeth densitometer (RD-914, using amber
filter).
Reproducibility of dots
Printed dots of the dynamically recorded image were observed by the naked
eye, and the evaluation results were expressed according to the following
ratings.
A: Good
B: Slightly poor
Deposition of Tailings
Copying was conducted on 20 sheets with B4 size having lengthwise stripes
by using a thermal facsimile UF-60, supplied by Matsushita Denso K.K., at
a copy-mode, and deposition of tailings on the thermal head was observed
by the naked eye. The Evaluation results were expressed according to the
following ratings.
A: Very good
B: Good
C: Slightly poor
The evaluation results are shown in Table 2.
EXAMPLE 4, COMPARATIVE EXAMPLES 1-5
By the same procedure as in Example 3, thermal recording sheets were made
except that latex (B) of bowl-shaped polymer particles (Example 4), and
the polymer particles shown in Table 1 (Comparative Examples 1-5) were
used instead of the latex (A) of bowl-shaped polymer particles, and the
thermal recording sheets were evaluated. The evaluation results are shown
in Table 2.
TABLE 1
Example No. Polymer particles
Example 4 Bowl-shaped polymer particles in latex (B)
prepared in Example 2
Comp. Ex. 1 Hollow spherical fine particles of styrene/
acrylate copolymer resin (average particle
diameter: about 0.5 .mu.m, vacant space ratio: 55%,
supplied by Nippon Zeon Co.)
Comp. Ex. 2 Hollow non-spherical fine particles of styrene/-
acrylate copolymer resin (average particle
diameter: about 1.5 .mu.m, vacant space ratio: 18%)
Comp. Ex. 3 Hollow spherical fine particles of polyvinylidene
chloride resin (average particle diameter: about
8 .mu.m, "Microsphere F-30", degassed, supplied by
Matsumoto Yushi Seiyaku. K. K.)
Comp. Ex. 4 Hollow spherical fine particles of alumino-
silicate (average particle diameter: about 20
.mu.m, Filite, supplied by Nippon Filite K. K.)
Comp. Ex. 5 Flat-shaped polymer particles prepared by the
method described in JP-A-H2-14222, Example 1
TABLE 2
Dynamic coloring Reproducibility Deposition of
Example No. density of dots tailings
Example 3 1.08 A A
Example 4 1.13 A A
Comp. Ex. 1 0.91 B B
Comp. Ex. 2 0.89 B B
Comp. Ex. 3 0.84 B C
Comp. Ex. 4 0.83 B C
Comp. Ex. 5 0.88 B C
INDUSTRIAL APPLICABILITY
The bowl-shaped polymer particles of the present invention has properties
advantageous for a heat insulating material, an organic pigment, an
opacifyinbg agent and others. Therefore, the polymer particles are useful
as ingredients to be incorporated in, for example, materials for forming a
heat-insulating intermediate layer of a thermal recording material, an
aqueous coating composition, and a coating color for paper.
Especially, an intermediate layer comprising the bowl-shaped polymer
particles, formed between a thermal recording layer and a support of a
thermal recording material, exhibits good heat-insulating property.
Therefore, utilization of heat energy of a thermal head and other elements
is improved and coloring sensitivity of a thermal recording layer is
enhanced. The bowl-shaped polymer particles exhibit an appropriate
elasticity in the intermediate layer, and thus, reproducibility of printed
dots is enhanced. Further, a problem of deposition of tailings on a
thermal head and sticking can be minimized in the thermal recording
material.
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